JPS60221519A - Production of austenitic stainless steel having high corrosion resistance and high strength - Google Patents

Abstract

PURPOSE:To improve corrosion resistance as well as strength, machineability for elongation, drawing, etc. of an austenitic stainless steel by controlling the upper limit for the content of C and N, making a solid solution of said steel at a specific temp., allowing V to coexist with Cr, Ni, etc. and heat-treating the steel under prescribed conditions. CONSTITUTION:The austenitic stainless steel consisting, by weight %, of 0.05- 0.15 C, 0.10-0.50 Si, 0.51-5.0 Mn, 18-25 Cr, 6.0<Ni<=10, 2-4 Mo, 0.05-0.25 V, 0.15-0.45 N and the balance iron is solubilized at 1,030-1,100 deg.C and is then quickly cooled. The steel can be thereby solubilized and strengthened while the decrease in the corrosion resistance occuring in addition of C and N and defects in the stage of producing the steel ingot are prevented. V is made to coexist with Cr, Ni, etc., by which the formation of Cr carbide is suppressed and the yield strength occuring in the dispersion and precipitation of the V carbonitride is improved. The quickly cooled steel is tempered to 250-500 deg.C, by which the intended product is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing high strength austenitic stainless steel with excellent corrosion resistance.

In recent years, the depletion of oil resources has come to be seen as a problem.
Against this background, in order to expand and stably secure energy resources, offshore oil fields are being developed, and acid gases such as hydrogen sulfide and carbon dioxide, which have traditionally been left untreated, are being removed on land. The so-called sour gas and sour oil that it contains has come to be mined. Materials and equipment related to the production of such energy resources inevitably come into contact with chlorides and acid gases, and therefore the steel materials used in materials and equipment in these fields must first have corrosion resistance. In addition, strength requirements are becoming more severe as wells become deeper, and low-temperature toughness is also required for use in cold regions.

Materials that can meet these demands include Ni-based alloys and T
I-based alloys, CO-based alloys, etc. are known, but these alloys are too expensive compared to conventional low alloy steels. Further, although austenitic stainless steel, for example, has been known as a relatively inexpensive material, it currently has poor corrosion resistance and strength, and is also poor in chloride corrosion resistance.

On the other hand, although martensitic stainless steel has almost satisfactory strength, it has the disadvantage of being extremely susceptible to sulfide stress corrosion cracking.

In general, methods for improving the yield strength of austenitic stainless steel include solid solution strengthening with C and N, precipitation strengthening, work strengthening, etc. However, precipitation strengthening and work strengthening have a negative effect on corrosion resistance. On the other hand, when the amount of C is increased, Cr carbide is produced and the corrosion resistance of the steel is deteriorated, and when the amount of N is increased, defects are likely to occur during the production of a steel ingot.

Therefore, as a result of intensive research to solve the above-mentioned problems, the inventors of the present invention have regulated the upper limit of the C and N content in austenitic stainless steel, and have determined that the addition of these elements will reduce the corrosion resistance of the steel. While aiming at solid solution strengthening while preventing the occurrence of defects during ingot production, V is reduced to Cr%N.
By coexisting with elements such as i, the formation of Cr carbides is suppressed, and furthermore, the yield strength is improved by the dispersion and precipitation of carbonitrides, thus improving both the yield strength and corrosion resistance of steel in particular. The present invention was achieved based on the discovery that a steel with improved machinability such as elongation and drawing can be obtained by heat-treating the steel according to predetermined conditions.

Therefore, an object of the present invention is to provide a high-strength austenitic stainless steel with excellent corrosion resistance, particularly in an environment containing chlorides and sulfides, and high strength. The purpose of the present invention is to provide a method for manufacturing austenitic stainless steel with excellent machinability.

The first method for producing highly corrosion-resistant and high-strength austenitic stainless copper according to the present invention is CO,05 to 0.1% by weight.
5%, Si0.10-0.50%, Mn 0.5-5.0%, Cr18-25%, Ni 6-10%, Mo2-4%, VO, 05-0.25%, NO , 15 to 0.45%, the balance being iron and unavoidable impurities, the austenitic stainless steel is solution treated at a temperature of 1030 to 1100"C, then rapidly cooled, and then tempered to a temperature of 250 to 500C. It is characterized by

Moreover, the second is CO, 05-0.15%, Si 0.10-0.50%, Mn 0.5-5.0%, Cr 18-25%, Ni 6-10%, in weight%. In addition to Mo 2-4%, VO, 05-0.25%, and NO, 15-0.45%, Nb 0005-0.50% and Ti 0.01-0°5
Austenitic stainless steel consisting of at least one type selected from 0%, the balance being iron and unavoidable impurities is solution treated at a temperature of 1030 to 1100°C, then rapidly cooled, and then tempered to a temperature of 250 to 500°C. It is characterized by

First, the reason for limiting the components in the austenitic stainless steel according to the present invention will be explained.

C is an austenite stabilizing element, and at the same time is an interstitial solid solution strengthening element that is effective in improving the strength of steel, including its yield strength. In the present invention, it is necessary to add 0.05% or more in order to precipitate fine carbonitrides in the coexistence with Ni and V and improve the yield strength and toughness of the steel. , 15%, Cr carbides are formed and corrosion resistance is deteriorated. Therefore, the C content is 0.05
~0.15%.

It is necessary to add Si in an amount of 0.10% or more as a deoxidizing agent for steel, but if it is added in excess, it increases the susceptibility to weld cracking and may also cause cracking during hot rolling. The upper limit is set to 0.50%.

Like Si, Mn is not only necessary as a deoxidizing agent for steel, but also should be added in an amount of 0.5% or more to increase the amount of solid solution of N, stabilize austenite, and improve weld cracking resistance. is necessary. However, when it exceeds 5%, problems such as impairing hot workability occur. Therefore, the Mn content is preferably in the range of 0.5 to 5%, and particularly preferably in the range of 2.0 to 4.0% from the viewpoint of improving weld cracking resistance.

In the present invention, Cr is an essential element for improving the corrosion resistance of steel, and is also an element necessary for increasing the solid solubility limit of N. However, if it is added excessively, the balance between austenite and ferrite will be disrupted, and in order to maintain the properties of the steel of the present invention, it will be necessary to add a large amount of expensive Ni, etc., which is undesirable.
The content is 18-25%.

Ni is an essential element for improving corrosion resistance and mechanical properties depending on the balance with Cr etc. For this purpose, it is necessary to add more than 6.0%, but on the other hand, When added excessively to Cr, the corrosion resistance deteriorates, so the upper limit is set at 10%.

Mo is an essential element for the corrosion resistance of steel, especially for preventing crevice corrosion and pitting corrosion.For this purpose, it is necessary to add 2% or more, but even if it is added in excess, the effect of improving corrosion resistance tends to be saturated. , which further increases the product price, so the upper limit is set to 4.
%.

(2) In the present invention, in order to improve the strength, toughness, and corrosion resistance of steel in a well-balanced manner, in particular, suppressing the formation of Cr carbides to improve corrosion resistance, and (2) improving the yield strength by dispersing precipitation of carbonitrides. for at least 0.05
% addition is required.

However, when added in excess, it promotes the formation of ferrite, disrupts the balance between austenite and ferrite, and deteriorates corrosion resistance. Therefore, the upper limit is set to 0.25%.

Like C, N is an austenite-forming element, and has the effect of improving the yield strength of steel by solid solution and improving toughness by forming fine carbonitrides. In order to effectively exhibit this effect, it is necessary to add 0.15% or more; however, if the amount decreases excessively, it will cause problems during the production of steel ingots, so the upper limit is set at 0.45%.

In addition to the above-described elements, at least one element selected from Nb and Ti can be added to the austenitic stainless steel according to the present invention.

Nb is known as an element that forms carbides and stabilizes C, but in steel with a high N content, it forms fine carbonitrides and improves both yield strength and toughness. In order to produce such an effect, it is necessary to add 0.05% or more, but when added in excess, weldability deteriorates, and Nb is an element that forms stable carbonitrides. This results in a decrease in the amount of solid solution C and N, which actually reduces the yield strength, and also forms huge carbonitrides, which significantly impairs toughness. Therefore, the upper limit is set to 0.50%.

Like Nb, Ti is also an element that forms very stable carbonitrides, and when added in the range of 0.01 to 0.50%, it improves the yield strength of steel, but when added in excess, it only improves the yield strength. However, the content is regulated as described above, since this leads to a decrease in toughness.

In the method of the present invention, steel having the above chemical composition is subjected to solution treatment at a temperature of 1030 to 1100°C, then rapidly cooled with water or oil cooling, and then
Tempering treatment to a temperature of 0°C.

In the method of the present invention, by solid solution treatment in the above temperature range, in addition to solid solution of Cr carbide, softening due to recrystallization,
It is possible to improve corrosion resistance, eliminate internal stress, etc. However, when the solution treatment temperature is lower than 1030° C., the solution treatment of Cr carbides is insufficient, and undissolved Cr carbides remain in the steel, which has a detrimental effect on the corrosion resistance of the steel. On the other hand, when performing solution treatment by heating to a high temperature exceeding 1100° C., carbides such as V and NbXTi are also melted, resulting in deterioration of yield strength. A particularly preferable solution treatment temperature range is 1040 to 1080°C.

The solution heating time needs to be increased in proportion to the thickness of the steel material, but in terms of -a, it may be determined at a rate of 1 hour per 25 mm thickness of the steel material. Cooling of steel materials that have undergone solution heating requires rapid cooling faster than oil cooling, that is, cooling with an average cooling rate of about 0.2"07 seconds or more, but in particular:
For example, in a temperature range of 900 to 500°C, Cr carbide is likely to precipitate at grain boundaries, so it is preferable to perform rapid cooling in this temperature range to ensure excellent corrosion resistance.

According to the method of the present invention, after the solution treatment and rapid cooling, the steel is tempered at a temperature of 250 to 500°C to remove thermal strain in the steel, thereby improving not only corrosion resistance and yield strength but also , elongation and reduction of area can be further improved. That is, by tempering to the above-mentioned temperature range, it is possible to alleviate the hardening effect due to the above-mentioned cold, remove thermal strain caused by solution treatment, and improve toughness. If the tempering temperature is less than 250° C., thermal strain cannot be sufficiently removed, so there is a risk that the stress corrosion cracking resistance of the steel will decrease. On the other hand, when the tempering temperature is high, exceeding 500° C., both corrosion resistance and mechanical properties deteriorate due to grain boundary precipitation of Cr carbide.

A more preferable tempering temperature range is 350 to 500°C, and an especially preferable temperature range is 400 to 500°C.

Conventionally, it has been known that austenitic stainless steel has the best corrosion resistance in a solution treated state, and that subsequent heating causes precipitation of Cr carbides and deteriorates corrosion resistance.

However, according to the method of the present invention, by tempering in the temperature range where low-temperature sensitization phenomenon occurs conventionally, mechanical stress such as elongation and drawing can be improved without impairing the excellent corrosion resistance of steel. As described above, according to the method for manufacturing austenitic stainless steel of the present invention, the upper limit of the content of C and N is regulated, and the addition of these elements reduces the corrosion resistance and improves the steel. While preventing the occurrence of defects during ingot production and aiming for solid solution strengthening,
By coexisting with elements such as Cr, the formation of Cr carbides is suppressed, and by dispersion strengthening of carbonitrides and a balance between solid solution strengthening and precipitation strengthening, excellent corrosion resistance is achieved in acidic environments. At the same time, an ausonitic stainless steel with excellent yield strength was obtained, and furthermore,
By tempering such stainless steel at a temperature range that is conventionally said to degrade corrosion resistance and mechanical properties, it is possible to further improve mechanical properties such as elongation and drawing while ensuring excellent corrosion resistance. It is.

The present invention will be explained below with reference to Examples.

Table 2 shows the mechanical properties of the steel shown in Example A in Table 1, which was heated to various temperatures and subjected to solution treatment. 103
It is shown that elongation is particularly improved by solution treatment at a temperature of 0°C or higher and then tempering at a temperature of 450°C. Table 2 also shows the results of stress corrosion cracking tests for each steel. It is recognized that corrosion resistance is similarly improved by solid solution treatment at a temperature of 1030° C. or higher.

Next, Table 3 shows the mechanical properties and stress corrosion cracking test results of the namesakes with grade numbers A, B, and C in Table 1, which were solution-treated, water-cooled, and then tempered to various temperatures. show.

However, steel A was solution treated at a temperature of 1040°C and then water cooled, and steel @B and C were solution treated at a temperature of 1050°C, water cooled, and tempered at 450°C. It is clear that by rapidly cooling after solution treatment and then tempering in a predetermined temperature range according to the method of the present invention, the elongation of the steel is significantly improved while maintaining its excellent corrosion resistance. .

In addition, in the present invention, 1, especially C, which contributes to improvement of yield strength.
-, S 1% Ni and N satisfy the following relationship in order to improve both the yield strength and corrosion resistance of austenitic stainless steel.

37≦100C%+2O3i% 10Ni%+6ON%≦4
9 When the value of the above formula is larger than 49, corrosion resistance deteriorates, while when it is smaller than 37, the balance between corrosion resistance and yield strength is lost, and it tends to be difficult to obtain an austenitic stainless steel with excellent both.

In the corrosion test, stress is applied to the test piece by U-bending, and NAC
After being immersed in E solution (5% salt solution + 0.5% acetic acid saturated with hydrogen sulfide gas at 41 atm), the corrosion rate was measured, and the presence or absence of cracks and holes were observed using a microscope (10 (B@)). The presence or absence of corrosion and crevice corrosion was determined.

Claims (2)

[Claims] (1) C, 0.05-0.15% by weight, Si 0.10-0.50%, Mn 0.5-5.0%, Cr 18-25%, Ni 6.0% over 10 %, Mo2 to 4%, VO, 05 to 0.25%, NO, 15 to 0.45%, and the balance iron and unavoidable impurities. A method for producing a highly corrosion-resistant, high-strength austenitic stainless steel, which comprises solution treatment followed by rapid cooling and then tempering at a temperature of 250 to 500°C. (2) CO, 05-0.15%, Sin, 10-0.50%, Mn 0.5-5.0%, Cr 18-25%, Ni5. Above Q% up to 10%, Mo2-4%, VO, 05-0.25%, and NO, 15-0.45%, plus NbO, 05-0.50% and TiO , 01-0.
10% austenitic stainless steel consisting of at least one type selected from 50%, the balance being iron and unavoidable impurities.
A method for producing highly corrosion resistant, high strength austenitic stainless steel, which comprises solution treatment at a temperature of 30 to 1100°C, followed by rapid cooling, and then tempering at a temperature of 250 to 500°C.